1. Field of the Invention
The present invention relates to a road friction coefficient estimating apparatus, more particularly to a system for controlling a transfer clutch of a four wheel drive vehicle and an apparatus for estimating a road friction coefficient of a motor vehicle including a two wheel drive vehicle.
2. Prior Arts
Four wheel drive motor vehicle whose front and rear wheels are both driven by the engine mounted thereon can have better running performance by varying the ratio of driving force distributed between the front and rear wheels. In recent years, driving force distribution control techniques in which the torque distribution ratio between the front and rear wheels is variably controlled by controlling a variable capacity transfer clutch depending on various vehicle parameters such as longitudinal acceleration, lateral acceleration, yaw rate or the like have been proposed.
The applicant of the present invention, in Japanese Patent Application Laid-open No. Toku-Kai-Hei 8-2274, proposes a technique wherein a road friction coefficient is estimated using an adaptive control theory and the torque distribution ratio between the front and rear wheels is calculated on the basis of the estimated road friction coefficient and other parameters associated with the vehicle such as vehicle traveling speed, actual yaw rate, input torque, target stability factor and the like.
According to the above technology, it is possible to enhance driving stability, turning ability, maneuverability of the vehicle under adverse road conditions or when the vehicle shows a critical behavior.
However, those prior arts have such problems as a complicated calculation process of the torque distribution ratio, a difficulty of the tuning to an actual vehicle due to complicated control parameters and the like.
Further, the above mentioned technique using the adaptive control theory has a problem of responsibility such as taking a long time to obtain an appropriate road friction coefficient in case of employing an inappropriate initial value for integration. Also, this technique has a problem of that it is difficult to estimate a proper road friction coefficient when the vehicle travels straight without turning the steering wheel, because this estimation method of road friction coefficient needs repeated inputs of yaw responses to rotational variations in the steering wheel.